Method for making light emitting diode

ABSTRACT

A method for making a light emitting diode is provided. In the method, a substrate having an epitaxial growth surface is provided. A buffer layer, a first semiconductor layer, an active layer, a second semiconductor layer are grown on the epitaxial growth surface in sequence. The first semiconductor layer, the active layer, and the second semiconductor layer constitute a source layer. A third optical symmetric layer, a metallic layer, a fourth optical symmetric layer, and a first optical symmetric layer are then disposed on a surface of the second semiconductor layer away from the substrate in the listed sequence. The substrate and the buffer layer are removed to expose the first semiconductor layer. A first electrode is applied on an exposed surface of the first semiconductor layer and a second electrode is applied to electrically connect with the second semiconductor layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 fromChina Patent Application No. 201210185723.4, filed on Jun. 7, 2012, inthe China Intellectual Property Office, the contents of which are herebyincorporated by reference. This application is related tocommonly-assigned applications entitled, “SEMICONDUCTOR STRUCTURE”,filed on Dec. 28, 2012, Ser. No. 13/729,284; “LIGHT EMITTING DIODE”,filed on Dec. 28, 2012, Ser. No. 13/729,310; “LIGHT EMITTING DIODE”,filed on Dec. 28, 2012, Ser. No. 13/729,351; “METHOD FOR MAKING LIGHTEMITTING DIODE”, filed on Dec. 28, 2012, Ser. No. 13/729,363;“SEMICONDUCTOR STRUCTURE”, filed on Dec. 28, 2012, Ser. No. 13/729,393;“METHOD FOR MAKING LIGHT EMITTING DIODE”, filed on Dec. 28, 2012, Ser.No. 13/729,427; “LIGHT EMITTING DIODE”, filed on Dec. 28, 2012, Ser. No.13/729,438; “METHOD FOR MAKING LIGHT EMITTING DIODE”, filed on Dec. 28,2012, Ser. No. 13/729,487; “LIGHT EMITTING DIODE”, filed on Dec. 28,2012, Ser. No. 13/729,506, the contents of the above commonly-assignedapplications are hereby incorporated by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to methods for making light emittingdiodes.

2. Description of Related Art

Semiconductor structures fabricated by gallium nitride for lightsources, such as blue, green, and white light sources, have longlifetime, high energy conversion efficiency, and green. Therefore, thesemiconductor structures are widely used as the light sources in largescreen color display systems, automotive lightening, traffic lights,multimedia displays, optical communication systems, and so on.

A conventional light emitting diode used as the light source includes anN-type semiconductor layer, a P-type semiconductor layer, and an activelayer located between the N-type semiconductor layer and a P-typesemiconductor layer. In an operation, a positive voltage and a negativevoltage are applied respectively to the P-type semiconductor layer andthe N-type semiconductor layer. Thus, holes in the P-type semiconductorlayer and electrons in the N-type semiconductor layer can enter theactive layer and combine with each other to emit visible light, and thevisible light is emitted from the light emitting diode. However, nearfield evanescent waves emitted from the active layer are internallyreflected inside the light emitting diode, so that a large portion ofthe light emitted from the active layer remain in the light emittingdiode, thereby degrading the light extraction efficiency.

What is needed, therefore, is to provide a method for making a lightemitting diode having a high light extraction efficiency.

BRIEF DESCRIPTION OF THE DRAWING

Many aspects of the present disclosure can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale, the emphasis instead being placed upon clearlyillustrating the principles of the present embodiments.

FIG. 1 is a schematic sectional view of an embodiment of a semiconductorstructure.

FIG. 2 is a schematic sectional view of one embodiment of asemiconductor structure.

FIG. 3 is a schematic sectional view of one embodiment of asemiconductor structure.

FIG. 4 is a schematic sectional view of one embodiment of asemiconductor structure.

FIG. 5 is a schematic sectional view of one embodiment of asemiconductor structure.

FIG. 6 is a schematic view of a second semiconductor layer having aplurality of three-dimensional nano-structures of the semiconductorstructure of FIG. 5.

FIG. 7 is a sectional view of the second semiconductor layer having theplurality of three-dimensional nano-structures of FIG. 6.

FIG. 8 is a scanning electron microscope image of the secondsemiconductor layer of FIG. 6.

FIG. 9 is a schematic view of one embodiment of a light emitting diode.

FIG. 10 is a schematic view of one embodiment of a light emitting diode.

FIG. 11 is a flowchart of one embodiment of a method for making thelight emitting diode of FIG. 10.

FIG. 12 is a schematic view of one embodiment of a light emitting diode.

FIG. 13 is a schematic view of one embodiment of a light emitting diode.

FIG. 14 is a schematic view of one embodiment of a light emitting diode.

FIG. 15 is a schematic view of one embodiment of a light emitting diode.

FIG. 16 is a schematic view of one embodiment of a light emitting diode.

FIG. 17 is a schematic view of one embodiment of a solar cell.

FIG. 18 is a schematic sectional view of one embodiment of a waveguide.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “another,” “an,” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean at leastone.

Referring to FIG. 1, one embodiment of a semiconductor structure 10includes a substrate 110, a buffer layer 116, a first semiconductorlayer 120, an active layer 130, a second semiconductor layer 140, athird optical symmetric layer 150, a metallic layer 160, a fourthoptical symmetric layer 170, a first optical symmetric layer 180, and asecond optical symmetric layer 190. The buffer layer 116, the firstsemiconductor layer 120, the active layer 130, the second semiconductorlayer 140, the third optical symmetric layer 150, the metallic layer160, the fourth optical symmetric layer 170, the first optical symmetriclayer 180, and the second optical symmetric layer 190 are stacked on asurface of the substrate 110 in sequence. A refractive index of thethird optical symmetric layer 150 and the fourth optical symmetric layer170 are substantially the same. The first semiconductor layer 120, theactive layer 130, and the second semiconductor layer 140 cooperativelyconstitute a source layer of the semiconductor structure 10. Adifference Δn₁ between a refractive index n₁ of the first opticalsymmetric layer 180 and an effective refractive index n₂ of the sourcelayer and the buffer layer 116 is less than or equal to 0.3, whereinΔn₁=|n₁−n₂|. A difference Δn₂ of a refractive index n₃ of the secondoptical symmetric layer 190 and a refractive index n₄ of the substrate110 is less than or equal to 0.1, wherein Δn₂=|n₃−n₄|. The semiconductorstructure 10 is an optical symmetric structure with the metallic layer160 as an optical symmetric center. The refractive indexes of twocomponents of the semiconductor structure 10 in the optical symmetricpositions to the optical symmetric center are close. The opticalsymmetric structure refers to two components in the optical symmetricposition that have a close refractive index. In one embodiment, the twocomponents in the optical symmetric position have a close refractiveindex and a close thickness.

The substrate 110 can be a transparent structure having an epitaxialgrowth surface 112 used to grow the first semiconductor layer 120. Theepitaxial growth surface 112 is a smooth surface. Oxygen and carbon areremoved from the surface 112. The substrate 110 can be a single layerstructure or a multiple layer structure. If the substrate 110 is asingle layer structure, the substrate 110 can be a single-crystalstructure. The single-crystal structure includes a crystal plane whichis used as the epitaxial growth surface 112. A material of the substrate110 can be silicon on insulator (SOI), LiGaO₂, LiAlO₂, Al₂O₃, Si, GaAs,GaN, GaSb, InN, InP, InAs, InSb, AlP, AlAs, AlSb, AlN, GaP, SiC, SiGe,GaMnAs, GaAlAs, GaInAs, GaAlN, GaInN, AlInN, GaAsP, InGaN, AlGaInN,AlGaInP, GaP:Zn, or GaP:N. If the substrate 110 is the multiple layerstructure, the substrate 110 should include at least one layer of thesingle-crystal structure mentioned previously. The material of thesubstrate 110 can be selected according to the first semiconductor layer120. In one embodiment, a lattice constant and thermal expansioncoefficient of the substrate 110 is similar to the first semiconductorlayer 120 thereof to improve a quality of the first semiconductor layer120. In one embodiment, the material of the substrate 110 is sapphire. Athickness, shape, and size of the substrate 110 are arbitrary and can beselected according to need.

The buffer layer 116 is disposed between and contacting the substrate110 and the first semiconductor layer 120. The buffer layer 116 isdisposed on the epitaxial growth surface 112 of the substrate 110 toimprove a quality of growth of the first semiconductor layer 120 via adecrease of the lattice mismatch. A thickness of the buffer layer 116can be in a range from about 10 nanometers to about 300 nanometers. Inone embodiment, the thickness of the buffer layer 116 is in a range fromabout 20 nanometers to about 50 nanometers. A material of the bufferlayer 116 can be GaN or AlN. In one embodiment, the thickness of thebuffer layer 116 is about 20 nanometers and the material of the bufferlayer 116 is a low temperature GaN. The buffer layer 116 is an optionalelement.

A thickness of the first semiconductor layer 120 can be in a range fromabout 1 micrometer to about 15 micrometers. The first semiconductorlayer 120 can be a doped semiconductor layer. The doped semiconductorlayer can be an N-type semiconductor layer or a P-type semiconductorlayer. A material of the N-type semiconductor layer can be at least oneof N-type GaN, N-type GaAs, and N-type cupric phosphide. A material ofthe P-type semiconductor layer can be at least one of P-type GaN, P-typeGaAs, and P-type cupric phosphide. The N-type semiconductor layer isconfigured to provide electrons, and the P-type semiconductor layer isconfigured to provide holes. In one embodiment, the material of thefirst semiconductor layer 120 is the N-type GaN doped with Si element,and the thickness of the first semiconductor layer 120 is about 1460nanometers.

The second semiconductor layer 140 can be the N-type semiconductor layeror the P-type semiconductor layer, and the types of the firstsemiconductor layer 120 and the second semiconductor layer 140 aredifferent to form a PN conjunction. The second semiconductor layer 140is disposed between the metallic layer 160 and the active layer 130. Athickness of the second semiconductor layer 140 is smaller than or equalto 210 nanometers. In one embodiment, the thickness of the secondsemiconductor layer 140 is in a range from about 5 nanometers to about20 nanometers. In one embodiment, the thickness of the secondsemiconductor layer 140 is in a range from about 10 nanometers to about20 nanometers. In one embodiment, the thickness of the secondsemiconductor layer 140 is in a range from about 10 nanometers to about30 nanometers. In one embodiment, the thickness of the secondsemiconductor layer 140 is about 10 nanometers, about 15 nanometers,about 20 nanometers, or about 30 nanometers. In one embodiment, thesecond semiconductor layer 140 is the P-type GaN doped with Mg element,and the thickness of the second semiconductor layer 140 is about 10nanometers.

The active layer 130 is a photon excitation layer to provide a locationfor the combination of the electrons and holes. Photons are produced inthe active layer 130 when the electrons and holes are combined. Theactive layer 130 can be one of a single layer quantum well film ormultilayer quantum well film. A material of the quantum well film can beat least one of GaInN, AlGaInN, GaAs, GaAlAs, GaInP, InAsP, and InGaAs.A thickness of the active layer 130 can be in a range from about 0.01micrometers to about 0.6 micrometers. In one embodiment, the material ofthe active layer 130 is a composition of InGaN and GaN, and thethickness of the active layer 130 is about 10 nanometers.

The effective refractive index n₂ of the source layer and the bufferlayer 116 can be in a range from about 2.0 to about 3.5 depending on thematerial of the source layer and the buffer layer 116. In oneembodiment, the material of the buffer layer 116 is the low temperatureGaN, the material of the first semiconductor layer 120 is the N-typeGaN, the material of the active layer 130 is the composition InGaN/GaN,and the material of the second semiconductor layer 140 is the P-typeGaN. With similar materials, the refractive indexes of the buffer layer116, the first semiconductor layer 120, the active layer 130, and thesecond semiconductor layer 140 are also similar. In one embodiment, theeffective refractive index n₂ of the source layer and the buffer layer116 is about 2.5.

A material of the metallic layer 160 can be selected according to wavelengths of light generated in the active layer 130. Lights with longwave lengths can be well extracted from the active layer 130. A materialof the metallic layer 160 can be an elemental metal or an alloy, such asgold, silver, aluminum, copper, gold-silver alloy, gold-aluminum alloy,or silver-aluminum alloy. The metallic layer 160 of the silver is goodfor extraction of purple light and blue light. The metallic layer 160 ofthe aluminum is good for extraction of wavelengths smaller than thewavelength of purple light. The metallic layer 160 of gold is good forextraction of red light and green light.

A thickness of the metallic layer 160 can be in a range from about 10nanometers to about 30 nanometers. In one embodiment, the thickness ofthe metallic layer 160 is in a range from about 20 nanometers to about30 nanometers. In one embodiment, the thickness of the metallic layer160 is in a range from about 15 nanometers to about 20 nanometers. Inone embodiment, the thickness of the metallic layer 160 is in a rangefrom about 10 nanometers to about 15 nanometers. In one embodiment, thethickness of the metallic layer 160 is about 10 nanometers, about 15nanometers, about 20 nanometers, about 25 nanometers, or about 30nanometers. In one embodiment, the thickness of the metallic layer 160is about 15 nanometers.

The function of the metallic layer 160 is as follows. First, near fieldevanescent waves generated by the active layer 130, when arriving themetallic layer 160, can be amplified and converted to a metallic plasmaby the metallic layer 160. The metallic plasma is then scattered by themetallic layer 160 and spreads around the metallic layer 160. Themetallic layer 160 is the optical symmetric center of the semiconductorstructure 10 and there are close refractive indexes in the opticalsymmetric positions of the semiconductor structure 10. Therefore, themetallic plasma can be uniformly distributed and uniformly propagated totwo opposite sides of the metallic layer 160. The metallic plasma can beuniformly exited passing through the second optical symmetric layer 190and the substrate 110. Second, there is a quantum well effect betweenthe metallic plasma and the active layer 130. The quantum well effectcan cause the active layer 130 to produce more photons and the producedphotons arrive at the metallic layer 160 to produce more metallicplasma. This interaction between the active layer 130 and the metalliclayer 160 can produce more photons from the active layer 130, therebyincreasing the extraction efficiency of the semiconductor structure 10.The less the distance between the metallic layer 160 and the activelayer 130, the better the interaction thereof, and the more uniformly acurrent in the second semiconductor layer 140 is distributed.

The difference Δn₁ between the refractive index n₁ of the first opticalsymmetric layer 180 and the effective refractive index n₂ of the sourcelayer and the buffer layer 116 is less than or equal to 0.3. In oneembodiment, the difference Δn₁ is less than or equal to 0.2 or 0.1. Thesmaller the difference Δn₁, the closer the refractive index n₁ of thefirst optical symmetric layer 180 and the effective refractive index n₂of the source layer and the buffer layer 116. Therefore, lightintensities extracted from the second optical symmetric layer 190 andthe substrate 110 are good. Therefore, the light can be uniformlyextracted from the semiconductor structure 10.

The refractive index n₁ of the first optical symmetric layer 180 can bein a range from about 2.0 to about 3.5. In one embodiment, therefractive index n₁ is in a range from about 2.2 to about 2.8. In oneembodiment, the refractive index n₁ is in a range from about 2.2 toabout 2.4. In one embodiment, the refractive index n₁ is in a range fromabout 2.4 to about 2.6. In one embodiment, the refractive index n₁ is ina range from about 2.6 to about 2.8. A material of the first opticalsymmetric layer 180 can be titanium dioxide, hafnium oxide, zirconia, orpolyimide. In one embodiment, the buffer layer 116 is the lowtemperature GaN, the material of the first semiconductor layer 120 isthe N-type GaN, the material of the active layer 130 is the compositionInGaN/GaN, the material of the second semiconductor layer 140 is theP-type GaN, the effective refractive index n₂ of the above fourcomponents is about 2.5, the material of the first optical symmetriclayer 180 is the titanium dioxide, and the refractive index n₁ is about2.55. The titanium dioxide has good transparency. Therefore, the lightcan be easily accessed.

A thickness difference between the first optical symmetric layer 180 andthe total thickness of the buffer layer 116 and the source layer can besmaller than or equal to 150 nanometers. The closer the thicknessbetween the first optical symmetric layer 180 and the total thickness ofthe buffer layer 116 and the source layer, the more uniform the emergentlight the semiconductor structure 10 can receive. The thickness of thefirst optical symmetric layer 180 can be in a range from about 1micrometer to about 2 micrometers. In one embodiment, the thickness ofthe buffer layer 116, the first semiconductor layer 120, the activelayer 130, and the second semiconductor layer 140 in the listed orderare about 20 nanometers, about 1460 nanometers, about 10 nanometers, andabout 10 nanometers. The total thickness of the buffer layer 116 and thesource layer is about 1500 nanometers and the thickness of the firstoptical symmetric layer 180 is about 1500 nanometers. In other words,the thickness of the first optical symmetric layer 180 and the totalthickness of the buffer layer 116 and the source layer can besubstantially the same.

The third optical symmetric layer 150 is disposed between the secondsemiconductor layer 140 and the metallic layer 160. The third opticalsymmetric layer 150 includes two opposite surfaces. One of the twoopposite surfaces directly contacts the second semiconductor layer 140and the other one of the two opposite surfaces directly contacts themetallic layer 160. The third optical symmetric layer 150 can preventthe metallic plasma generated by the metallic layer 160 from beingconverted to heat. A propagation constant of the metallic plasma under aguided wave mode is a complex number including a real part and animaginary part. If the imaginary part is large, the metallic plasma iseasily converted to heat. A material with a low refractive index can beselected as the material of the third optical symmetric layer 150 toreduce the real part and the imaginary part at the same time. Thus, aheat consumption of the metallic plasma can be reduced and the metallicplasma can travel farther. Therefore, the extraction efficiency of thesemiconductor structure 10 can be increased. A refractive index of thethird optical symmetric layer 150 can be in a range from about 1.2 toabout 1.5. In one embodiment, the refractive index is in a range fromabout 1.3 to about 1.4. In one embodiment, the refractive index is in arange from about 1.4 to about 1.5. A material of the third opticalsymmetric layer 150 can be silicon dioxide, magnesium fluoride, orlithium fluoride. In one embodiment, the material of the third opticalsymmetric layer 150 is the silicon dioxide, and the refractive index ofthe third optical symmetric layer 150 is about 1.5.

A thickness of the third optical symmetric layer 150 can be in a rangefrom about 5 nanometers to about 40 nanometers. In one embodiment, thethickness of the third optical symmetric layer 150 is in a range fromabout 5 nanometers to about 10 nanometers. In one embodiment, thethickness of the third optical symmetric layer 150 is in a range fromabout 10 nanometers to about 20 nanometers. In one embodiment, thethickness of the third optical symmetric layer 150 is in a range fromabout 20 nanometers to about 30 nanometers. In one embodiment, thethickness of the third optical symmetric layer 150 is in a range fromabout 30 nanometers to about 40 nanometers. In one embodiment, thethickness of the third optical symmetric layer 150 is about 20nanometers.

The fourth optical symmetric layer 170 is disposed between and directlycontacts the metallic layer 160 and the first optical symmetric layer180. A material and a thickness of the fourth optical symmetric layer170 are the same as the material and the thickness of the third opticalsymmetric layer 150. Therefore, the metallic plasma can uniformly traveltoward the substrate 110 and the second optical symmetric layer 190. Thelower refractive index of the third optical symmetric layer 150 and thefourth optical symmetric layer 170, the better extraction efficiency ofthe semiconductor structure 10. A function of the fourth opticalsymmetric layer 170 is similar to the third optical symmetric layer 170which can prevent the metallic plasma from being converted to heat,except that the metallic plasma can travel through the fourth opticalsymmetric layer 170.

The refractive index n₃ of the second optical symmetric layer 190 andthe refractive index n₄ are close in value such that the metallic plasmacan uniformly travel toward both the substrate 110 and the secondoptical symmetric layer 190. The difference Δn₂ of the refractive indexn₃ of the second optical symmetric layer 190 and the refractive index n₄of the substrate 110 is less than or equal to 0.1, wherein Δn₂=|n₃−n₄|.The closer the refractive index n₃ and n₄, the better the extractionefficiency of the semiconductor structure 10. The refractive index n₃ ofthe second optical symmetric layer 190 can be in a range from about 1.7to about 1.8 depending on a material of the second optical symmetriclayer 190. The material of the second optical symmetric layer 190 can bethe same as the material of the substrate 110. A thickness of the secondoptical symmetric layer 190 can be in a range from about 30 nanometersto about 80 nanometers. In one embodiment, the thickness of the secondoptical symmetric layer 190 is in a range from about 40 nanometers to 60nanometers. In one embodiment, the material of the second opticalsymmetric layer 190 is sapphire, and the thickness thereof is about 50nanometers. The second optical symmetric layer 190 is an optionalcomponent.

In the semiconductor structure 10, the substrate 110 is opticallysymmetric to the second optical symmetric layer 190, the first opticalsymmetric layer 180 is optically symmetric to the source layer addedwith the buffer layer 116 (if the buffer layer is included), and thethird optical symmetric layer 150 is optically symmetric to the fourthoptical symmetric layer 170. Some of the two components in the symmetricposition are optional in the semiconductor structure 10, such as thesubstrate 110 and the second optical symmetric layer 190, and the thirdoptical symmetric layer 150 and the fourth optical symmetric layer 170.

Referring to FIG. 2, one embodiment of a semiconductor structure 20 isprovided. The semiconductor structure 20 is similar to the semiconductorstructure 10, except that there is no the third optical symmetric layerand the fourth optical symmetric layer in the semiconductor structure20. In the semiconductor structure 20, the metallic layer 160 isdirectly disposed on a surface of the second semiconductor layer 140which is far away from the substrate 110, and the first opticalsymmetric layer 180 is directly disposed on a surface of the metalliclayer 160 which is far away from the substrate 110. The metallic layer160 can cover the entire surface of the second semiconductor layer 140.The metallic layer 160 is closer to the active layer 130 in thesemiconductor structure 20 than in the semiconductor structure 10.Therefore, there is a strong interaction between the metallic layer 160and the active layer 130 which can make the active layer 130 generatemore photons and the metallic layer 160 generate more metallic plasma.

Referring to FIG. 3, one embodiment of a semiconductor structure 30 isprovided. The semiconductor structure 30 is similar to the semiconductorstructure 10, except that there is no substrate, the buffer layer, andthe second optical symmetric layer in the semiconductor structure 30. Arefractive index difference between the first optical symmetric layer180 and the source layer is less than or equal to 0.3 in thesemiconductor structure 30, and the metallic layer is still the opticalsymmetric center of the semiconductor structure 30. The refractive indexof the source layer is an effective refractive index of the firstsemiconductor layer 120, the active layer 130, and the secondsemiconductor layer 140. In one embodiment, the refractive index of thesource layer is about 2.5.

An exposed surface of the first semiconductor layer 120 and an exposedsurface of the first optical symmetric layer 180 are two light emittingsurfaces of the semiconductor structure 30. Optical symmetric componentson the two sides of the metallic layer 160 have a close refractiveindex. Therefore, lights generated from the active layer 130 can beuniformly extracted from the two light emitting surfaces. The opticalsymmetric components in the semiconductor structure 30 refer to thesource layer and the first optical symmetric layer 180, and the thirdoptical symmetric layer 150 and the fourth optical symmetric layer 170.

Referring to FIG. 4, one embodiment of a semiconductor structure 40 isprovided. The semiconductor structure 40 is similar to the semiconductorstructure 10, except that there is no substrate, buffer layer, secondoptical symmetric layer, third optical symmetric layer, and fourthoptical symmetric layer. The metallic layer 160 is directly disposed onthe surface of the second semiconductor layer 140, and the first opticalsymmetric layer 180 is directly disposed on the surface of the metalliclayer 160 which is far away from the second semiconductor layer 140.

Referring to FIG. 5, one embodiment of a semiconductor structure 50 isprovided. The semiconductor structure 50 is similar to the semiconductorstructure 10, except that the semiconductor structure 50 includes aplurality of three-dimensional nano-structures 143. Each of theplurality of three-dimensional nano-structures 143 is a protrudingstructure and has an M-shaped cross-section.

At least one component of the substrate 110, the buffer layer 116, thefirst semiconductor layer 120, the active layer 130, the secondsemiconductor layer 140, the third optical symmetric layer 150, themetallic layer 160, the fourth optical symmetric layer 170, the firstoptical symmetric layer 180, and the second optical symmetric layer 190can include the plurality of three-dimensional nano-structures 143. Theat least one component includes two opposite surfaces. The plurality ofthree-dimensional nano-structures 143 can be disposed on at least onesurface of the two opposite surfaces of the at least one component. Twoadjacent components including the plurality of three-dimensionalnano-structures 143 of the semiconductor structure 50 can be meshed witheach other via the plurality of three-dimensional nano-structures 143.The two adjacent components refer to two components contacting eachother in the semiconductor structure 50. In one embodiment, theplurality of three-dimensional nano-structures 143 are disposed on thesecond semiconductor layer 140 away from the substrate 110, the thirdoptical symmetric layer 150, the metallic layer 160, the fourth opticalsymmetric layer 170, the first optical symmetric layer 180, and thesecond optical symmetric layer 190 of the semiconductor structure 50,and the two adjacent components are meshed with each other via thethree-dimensional nano-structures 143. The plurality ofthree-dimensional nano-structures 143 on different components of thesemiconductor structure 50 are aligned and arranged the same.

Referring to FIGS. 5-8, the plurality of three-dimensional structures143 disposed on a surface of the second semiconductor layer 140 awayfrom the substrate 110 will be selected as an example to be introducedas follows. The plurality of three-dimensional structures 143 arearranged as a pattern. The plurality of three-dimensionalnano-structures 143 can be arranged side by side. Each of thethree-dimensional nano-structures 143 can extend along a straight line,a curvy line, or a polygonal line. The extending direction issubstantially parallel with the surface of the second semiconductorlayer 140. Two adjacent three-dimensional nano-structures 143 arearranged a certain distance apart from each other. The distance can bein a range from about 0 nanometers to about 200 nanometers. Theextending direction of each of the plurality of three-dimensionalnano-structures 143 can be fixed or varied. If the extending directionis fixed, each of the plurality of three-dimensional nano-structures 143extends along a straight line, otherwise each of the plurality ofthree-dimensional nano-structures 143 extends along a polygonal line ora curvy line. The cross-section of each of the plurality ofthree-dimensional nano-structures 143 is M-shaped. In one embodiment,each of the plurality of three-dimensional nano-structures 143 is abar-shaped protruding structure. The plurality of three-dimensionalnano-structures 143 are substantially parallel with each other andextending along the straight line. The plurality of three-dimensionalnano-structures 143 are substantially uniformly and equidistantlydistributed on the entire surface of the second semiconductor layer 140.

Each of the plurality of three-dimensional nano-structures 143 extendsfrom one side of the second semiconductor layer 140 to an opposite sidealong an X direction. A Y direction is substantially perpendicular tothe X direction and substantially parallel with the surface of thesecond semiconductor layer 140. Each of the three-dimensionalnano-structures 143 is a double-peak structure including a first peak1132 and a second peak 1134. The cross-section of the double-peakstructure is in a shape of an “M”. The first peak 1132 and the secondpeak 1134 extend substantially along the X direction. The first peak1132 includes a first surface 1132 a and a second surface 1132 b. Thefirst surface 1132 a and the second surface 1132 b intersect to form anintersection line and an included angle of the first peak 1132. Theintersection line can be a straight line, a curvy line, or a polygonalline. The included angle of the first peak 1132 is greater than 0degrees and smaller than 180 degrees. The first surface 1132 a and thesecond surface 1132 b can be planar, curvy, or wrinkly. In oneembodiment, the first surface 1132 a and the second surface 1132 b aresubstantially planar. An angle between the first surface 1132 a and thesurface of the second semiconductor layer 140 which intersects with thefirst surface 1132 a is greater than 0 degrees and less than or equal to90 degrees.

The second peak 1134 includes a third surface 1134 a and a fourthsurface 1134 b. The structure of the second peak 1134 is substantiallythe same as that of the first peak 1132. The fourth surface 1134 bincludes a side intersecting the third surface 1134 a to form anincluded angle of the second peak 1134, and extends to intersect thesecond surface 1132 b of the first peak 1132 to define a first groove1136. A second groove 1138 is defined between two adjacentthree-dimensional nano-structures 143. The second groove 1138 is definedby the third surface 1134 a of the second peak 1134 and the firstsurface 1132 a of the first peak 1132 of the adjacent three-dimensionalnano-structures 143.

The first peak 1132 and the second peak 1134 protrude out of the secondsemiconductor layer 140. A height of the first peak 1132 and the secondpeak 1134 can be in a range from about 150 nanometers to about 200nanometers. The height of the first peak 1132 can be substantially equalto the height of the second peak 1134. Cross-sections of the first peak1132 and 1134 can be trapezoidal or triangular, and shapes of the firstpeak 1132 and the second peak 1134 can be substantially the same. In oneembodiment, the cross-sections of the first peak 1132 and the secondpeak 1134 are triangular.

An extending direction of the first groove 1136 is substantiallyparallel to the extending direction of the first peak 1132 and thesecond peak 1134. The cross-section of the first groove 1136 can beV-shaped. A depth of the first groove 1136 in the differentthree-dimensional nano-structures 143 is substantially the same. Thedepth of the first groove 1136 is defined as a distance between thehighest point of the first peak 1132 and the lowest point of the firstgroove 1136. The depth of the first groove 1136 is less than the heightof the first peak 1132 and the second peak 1134.

The second groove 1138 extends substantially along the extendingdirection of the plurality of three-dimensional nano-structures 143. Across-section of the second groove 1138 can be V-shaped or an inversetrapezium. The cross-section of the second groove 1138 is substantiallythe same along the extending direction. Depths of the second grooves1138 between each two adjacent three-dimensional nano-structures 143 aresubstantially the same. A depth of the second groove 1138 is defined asa distance between the highest point and the lowest point of the secondgroove 1138. The depth of the second groove 1138 is greater than thedepth of the first groove 1136, and the ratio between the depth of thefirst groove 1136 and the depth of the second groove 1138 ranges fromabout 1:1.2 to about 1:3. The depth of the first groove 1136 ranges fromabout 30 nanometers to about 120 nanometers, and the depth of the secondgroove 1138 ranges from about 90 nanometers to about 200 nanometers. Inone embodiment, the depth of the first groove 1136 is about 80nanometers, and the depth of the second groove 1138 is about 180nanometers.

The plurality of three-dimensional nano-structures 143 disposed on thesecond semiconductor layer 140 and the second semiconductor layer 140can be an integrated structure. In other words, the plurality ofthree-dimensional nano-structures 143 can be a part of the secondsemiconductor layer 140 and the plurality of three-dimensionalnano-structures 143 as a patterned structure can be formed by etchingone surface of the second semiconductor layer 140. If the secondsemiconductor layer 140 and the plurality of three-dimensionalnano-structures 143 are the integrated structure, the thickness of thesecond semiconductor layer 140 is greater than the heights of the firstand second peaks and the depths of the first and second grooves. Thisprinciple is adapted to other components of the semiconductor structure50 with the plurality of three-dimensional nano-structures 143.

The semiconductor structure 50 is still the optical symmetric structurehaving the metallic layer 160 as the symmetric center. The twocomponents each with the plurality of three-dimensional nano-structures143 on the two opposite sides of the metallic layer 160 have arefractive index close in value and a thickness close in value.

Referring back to FIG. 5, each layer of the second semiconductor layer140, the third optical symmetric layer 150, the metallic layer 160, thefourth optical symmetric layer 170, the first optical symmetric layer180, and the second optical symmetric layer 180 includes the pluralityof three-dimensional nano-structures 143. Two adjacent layers are meshedwith each other via the plurality of three-dimensional nano-structures143. More specifically, a position relationship between the secondsemiconductor layer 140 and the third optical symmetric layer 150 istaken as an example. The second semiconductor layer 140 directlycontacts the third optical symmetric layer 150 via the plurality ofthree-dimensional nano-structures 143. The first and second grooves ofthe three-dimensional nano-structures 143 on the third optical symmetriclayer 150 are meshed with and filled the first and second peaks of thethree-dimensional nano-structures 143 on the second semiconductor layer140. In addition, the first and second peaks of the three-dimensionalnano-structures 143 on the third optical symmetric layer 150 are meshedwith and filled the first and second grooves of the three-dimensionalnano-structures 143 on the second semiconductor layer 140. Thethree-dimensional nano-structures 143 meshed on other three-dimensionalnano-structures 143 can be directly grown therefrom. The plurality ofthree-dimensional nano-structures 143 disposed on each layer of thesemiconductor structure 50 have the same distribution and alignment.

The semiconductor structure 50 has the following advantages inoperation. First, an extracting angle of the photons formed by theactive layer 130 can be changed to avoid being reflected when enteringthe plurality of three-dimensional nano-structures 143. Thus, the lightextraction efficiency can be increased. Second, if the three-dimensionalnano-structures 143 are formed on the metallic layer 160, morescattering lights can be formed on the surface of the metallic layer160. Thus, the metallic plasma can be more easily released from themetallic layer 160. Therefore, a luminous efficiency of thesemiconductor structure 50 can be increased. Third, if the plurality ofthree-dimensional nano-structures 143 are formed on at least one surfaceof the active layer 130, a contact area between the active layer 130 andthe first semiconductor layer 120 or the second semiconductor layer 140can be enlarged. The electron-hole recombination density is furtherincreased, and the light extraction efficiency of semiconductorstructure 50 can be improved.

The above semiconductor structures 10, 20, 30, 40, and 50 can be widelyused in solar batteries, lasers, and light emitting devices (LED).

Referring to FIG. 9, one embodiment of an LED 60 includes the substrate110, the buffer layer 116, the first semiconductor layer 120, the activelayer 130, the second semiconductor layer 140, the third opticalsymmetric layer 150, the metallic layer 160, the fourth opticalsymmetric layer 170, the first optical symmetric layer 180, the secondoptical symmetric layer 190, a first electrode 124, and a secondelectrode 144. The buffer layer 116, the first semiconductor layer 120,the active layer 130, the second semiconductor layer 140, the thirdoptical symmetric layer 150, the metallic layer 160, the fourth opticalsymmetric layer 170, the first optical symmetric layer 180, and thesecond optical symmetric layer 190 are stacked on the surface of thesubstrate 110 in sequence. The first electrode 124 is disposed on asurface of the first semiconductor layer 120 away from the substrate 110and electrically connected with the first semiconductor layer 120. Thesecond electrode 144 is disposed on a surface of the secondsemiconductor layer 140 away from the substrate 110 and electricallyconnected with the second semiconductor layer 140. The firstsemiconductor layer 120, the active layer 130, and the secondsemiconductor layer 140 constitute the source layer. The difference Δn₁between the refractive index n₁ of the first optical symmetric layer 180and the effective refractive index n₂ of the source layer and the bufferlayer 116 is less than or equal to 0.3, wherein Δn₁=|n₁−n₂|. Thedifference Δn₂ between the refractive index n₃ of the second opticalsymmetric layer 190 and the refractive index n₄ of the substrate 110 isless than or equal to 0.1, wherein Δn₂=|n₃−n₄|.

The first electrode 124 and the second electrode 144 are disposed on thesame side of the first semiconductor layer 120.

The surface of the first semiconductor layer 120 away from the substrate110 is defined into a first region and a second region. The active layer130 and the second semiconductor layer 140 are stacked on the firstregion in sequence. The active layer 130 covers the first region of thefirst semiconductor layer 120. The first electrode 124 is disposed onthe second region which is uncovered by the active layer 130 andelectrically connected with the first semiconductor layer 120.

The surface of the second semiconductor layer 140 away from thesubstrate 110 is defined into a third region and a fourth region. Thethird optical symmetric layer 150 or the metallic layer 160 is disposedon and covers the third region. The second electrode 144 is disposed onthe fourth region which is uncovered by the metallic layer 160 or thethird optical symmetric layer 150 and electrically connected with thesecond semiconductor layer 140.

The first electrode 124 can be an N-type electrode or a P-type electrodewhich is consistent with the first semiconductor layer 120. The secondelectrode 144 can be an N-type electrode or a P-type electrode which isconsistent with the second semiconductor layer 140.

The first electrode 124 or the second electrode 144 can be a layeredstructure including at least one layer. A thickness of the firstelectrode 124 or the second electrode 144 can be in a range from about0.01 micrometers to about 2 micrometers. A material of the firstelectrode and the second electrode can be the same, such as titanium(Ti), silver (Ag), aluminum (Al), nickel (Ni), gold (Au) or an alloythereof. In one embodiment, the second electrode 144 is a P-typeelectrode including two layers, which are a titanium layer with thethickness of about 15 nanometers and a gold layer with the thickness ofabout 200 nanometers. The first electrode 124 is an N-type electrodeincluding two layers, a titanium layer with the thickness of about 15nanometers and a gold layer with the thickness of about 100 nanometers.

In an operation of the LED 60, a voltage is applied to the firstsemiconductor layer 120 via the first electrode 124 and the secondsemiconductor layer 140 via the second electrode 144. The photons arethen generated from the active layer 130 as the near field evanescentwaves reaches the metallic layer 160. The metallic plasma is thengenerated from the metallic layer 160, spreads around, and coupled intoan emergent light emitted out from the second optical symmetric layer190. This process can increase the light extraction efficiency of theLED 60. In the process, the quantum well effect between the metallicplasma and the active layer 130 can cause the active layer 130 toproduce more photons and the produced photons arrive at the metalliclayer 160 to produce more metallic plasma. Therefore, a luminousefficiency of the LED 60 can be increased. The LED 60 is an opticalsymmetric structure with the metallic layer 160 as the symmetric center.The refractive indexes of two components of the LED 60 in the symmetricpositions to the symmetric center are close. Therefore, lights can beuniformly extracted from the second optical symmetric layer 190 and thesubstrate 110. In addition, the third optical symmetric layer 150 andthe fourth optical symmetric layer 170 disposed on the two oppositesides of the metallic layer 160 have a low refractive index. Therefore,the metallic plasma generated from the metallic layer 160 can avoidbeing converted to heat. Instead, a majority of the metallic plasma canbe converted to light extracted out. Therefore, the light extractionefficiency of the LED 60 can be further increased.

Referring to FIG. 10, one embodiment of an LED 70 is provided. The LED70 is substantially the same as the LED 60, except that there are nothird optical symmetric layer and fourth optical symmetric layer in theLED 70.

Referring to FIG. 11, one embodiment of a method for making the LED 70includes the following steps:

-   -   S710, providing the substrate 110 with the epitaxial growth        surface 112;    -   S720, growing the buffer layer 116 on the epitaxial growth        surface 112, the first semiconductor layer 120 on the surface of        the buffer layer 116, the active layer 130 on the surface of the        first semiconductor layer 120, and the second semiconductor        layer 140 on the surface of the active layer in sequence;    -   S730, forming the metallic layer 160 on the surface of the        second semiconductor layer 140 away from the substrate 110;    -   S740, disposing the first optical symmetric layer 180 on the        surface of the metallic layer 160 and the second optical        symmetric layer 190 on the surface of the first optical        symmetric layer 180 in sequence, the difference Δn₁ between the        refractive index n₁ of the first optical symmetric layer 180 and        the effective refractive index n₂ of the source layer and the        buffer layer 116 being less than or equal to 0.3, wherein        Δn₁=|n₁−n₂|, the difference Δn₂ between the refractive index n₃        of the second optical symmetric layer 190 and the refractive        index n₄ of the substrate 110 is less than or equal to 0.1, and        Δn₂=|n₃−n₄|; and    -   S750, applying the first electrode 124 to be electrically        connected with the first semiconductor layer 120, and the second        electrode 144 electrically connected with the second        semiconductor layer 140.

In step S720, the buffer layer 116, the first semiconductor layer 120,the active layer 130, and the second semiconductor layer 140 can grow bymethods of molecular beam epitaxy (MBE), chemical beam epitaxy (CBE),reduced pressure epitaxy, selective epitaxy, liquid phase depositionepitaxy (LPE), metal organic vapor phase epitaxy (MOVPE), super vacuumchemical vapor deposition, hydride vapor phase epitaxy (HVPE), metalorganic chemical vapor deposition (MOCVD), or combinations thereof. Inone embodiment, the buffer layer 116, the first semiconductor layer 120,the active layer 130, and the second semiconductor layer 140 grow by themethod of MOCVD.

A low temperature GaN layer is selected as the buffer layer 116. Anammonia gas as a nitrogen source, a hydrogen gas as a carrier gas, andtrimethyl gallium (TMGa) or triethyl gallium (TEGa) as a gallium sourceto grow the low temperature GaN layer in a reactor under a lowtemperature.

An N-type GaN layer is selected as the first semiconductor layer 120,the ammonia gas as the nitrogen source, the TMGa or TEGa as the galliumsource, silane as a Si source, and the hydrogen gas as the carrier gasto grow the N-type GaN layer in the reactor.

A process for growing the active layer 130 is substantially the same asthe process of growing the first semiconductor layer 120, except thatthe trimethyl indium is selected as an indium source. The active layer130 is grown on the first region of the first semiconductor layer 120 byusing a photoresist as a mask. The second region of the active layer 130is exposed to dispose the first electrode 124.

After the active layer 130 has been grown, a magnesocene (Cp₂Mg) is usedas a magnesium source to grow the second semiconductor layer 140. Thethickness of the second semiconductor layer 140 is in a range from about5 nanometers to about 60 nanometers by controlling a growing timeperiod. Selectively, a thick second semiconductor layer 140 can beformed by the MOCVD method and then etched or grinded to control thethickness of the second semiconductor layer 140 in the range from about5 nanometers to about 60 nanometers.

In one embodiment, the thickness of the buffer layer 116 is about 20nanometers, the thickness of the first semiconductor layer 120 is about1460 nanometers, the thickness of the active layer 130 is about 10nanometers, the thickness of the second semiconductor layer 140 is about10 nanometers, and the total thickness thereof is about 1500 nanometers.

In step S730, the metallic layer 160 can be formed by a physical vapordeposition method depending on the selected material of the metalliclayer 160, such as evaporation or sputtering. In one embodiment, asilver layer formed by evaporation is used as the metallic layer 160. Athickness of the silver layer is about 10 nanometers.

The metallic layer 160 is formed only on the third region of the secondsemiconductor layer 140 by using the photoresist as the mask. The fourthregion is exposed to dispose the second electrode 144.

In step S740, the first optical symmetric layer 180 can be formed byevaporation or sputtering which depends on the material of the firstoptical symmetric layer 180. The material of the first optical symmetriclayer 180 and the substrate 110 can be the same. The refractive index n₁of the first optical symmetric layer 180 is in a range from about 2.2 toabout 2.8. The thickness of the first optical symmetric layer 180 is ina range from about 1 micrometer to about 2 micrometers. In oneembodiment, the first optical symmetric layer 180 is formed byevaporating titanium dioxide, the refractive index the first opticalsymmetric layer 180 is about 2.55, and the thickness is about 1500micrometers consistent with the total thickness of the buffer layer 116and the source layer.

The second optical symmetric layer 190 can be formed by the evaporationor sputtering. The material of the second optical symmetric layer 190and the substrate can be the same. The thickness of the second opticalsymmetric layer 190 is in a range from about 30 nanometers to about 80nanometers. In one embodiment, the material of the second opticalsymmetric layer 190 is aluminum oxide, and the thickness is about 50nanometers.

One embodiment of a method for making the LED 60 includes the followingsteps:

-   -   S810, providing the substrate 110 with the epitaxial growth        surface 112;    -   S820, growing the buffer layer 116 on the epitaxial growth        surface 112, the first semiconductor layer 120 on the surface of        the buffer layer 116, the active layer 130 on the surface of the        first semiconductor layer 120, and the second semiconductor        layer 140 on the surface of the active layer in sequence;    -   S830, forming the third optical symmetric layer 150 on the        surface of the second semiconductor layer 140 away from the        substrate 110, and the refractive index of the third optical        symmetric layer 150 is in a range from about 1.2 to about 1.5;    -   S840, locating the metallic layer 160 on the surface of the        third optical symmetric layer 150 away from the substrate 110;    -   S850, disposing the fourth optical symmetric layer 170 on the        surface of the metallic layer 160 away from the substrate 110,        the refractive index of the fourth optical symmetric layer 170        being in a range from about 1.2 to about 1.5;    -   S860, disposing the first optical symmetric layer 180 on the        surface of the fourth optical symmetric layer 170, and the        second optical symmetric layer 190 on the surface of the first        optical symmetric layer 180 in sequence, the difference Δn₁        between the refractive index n₁ of the first optical symmetric        layer 180 and the effective refractive index n₂ of the source        layer and the buffer layer 116 being less than or equal to 0.3,        wherein Δn₁=|n₁−n₂|, the difference Δn₂ between the refractive        index n₃ of the second optical symmetric layer 190 and the        refractive index n₄ of the substrate 110 is smaller or equal to        0.1, and Δn₂=|n₃−n₄|; and    -   S870, applying the first electrode 124 electrically connected        with the first semiconductor layer 120, and the second electrode        144 electrically connected with the second semiconductor layer        140.

The method for making the LED 60 is substantially the same as the methodfor making the LED 70, except that the third optical symmetric layer 150is formed between the metallic layer 150 the second semiconductor layer140, and the fourth optical symmetric layer 170 is formed between themetallic layer 150 and the first optical symmetric layer 180.

The third optical symmetric layer 150 and the fourth optical symmetriclayer 170 can use the same material and can be formed by methods ofelectron beam evaporation, magnetron sputtering, or chemical vapordeposition. In one embodiment, a silicon oxide is deposited on thesurface of the second semiconductor layer 140 to form the third opticalsymmetric layer 150 by the chemical vapor deposition method. Thethickness of the third optical symmetric layer 150 is about 30nanometers and the fourth optical symmetric layer 170 is about 30nanometers thick.

The third optical symmetric layer 150 and the fourth optical symmetriclayer 170 disposed on the two opposite sides of the metallic layer 160can prevent the metallic plasma from being converted into heat.

Referring to FIG. 12, one embodiment of an LED 80 is provided. The LED80 is substantially the same as the LED 70, except that the LED 80includes the plurality of three-dimensional nano-structures 143.Disposing ways and positions of the plurality of three-dimensionalnano-structures 143 of the LED 80 and the semiconductor structure 50 aresubstantially same. In one embodiment, the surface of the secondsemiconductor layer 140 away from the substrate 110 is patterned to formthe plurality of three-dimensional nano-structures 143 and the otherlayers disposed on the surface of the second semiconductor layer 140away from the substrate 110 are all patterned forming the plurality ofthree-dimensional nano-structures 143. The plurality ofthree-dimensional nano-structures 143 on different layers of the LED 80are aligned and arranged substantially the same. The second electrode144 directly contacts a part of the plurality of three-dimensionalnano-structures 143 of the second semiconductor layer 140.

The LED 80 has the following advantages in operation. First, theextracting angle of the photons formed by the active layer 130 can bechanged to avoid being reflected when entering the plurality ofthree-dimensional nano-structures 143. Thus, the light extractionefficiency can be increased. Second, the plurality of three-dimensionalnano-structures 143 are formed on at least one surface of the activelayer 130, and the contact area between the active layer 130 and thefirst semiconductor layer 120 or the second semiconductor layer 140 canbe enlarged. The electron-hole recombination density is furtherincreased, and the light extraction efficiency of the LED 80 can beimproved. Third, the second electrode 144 directly contacts a part ofthe plurality of three-dimensional nano-structures 143 of the secondsemiconductor layer 140. Therefore, a contact area between the secondelectrode 144 and the second semiconductor layer 140 can be enlarged. Acurrent applied via the second electrode 144 can be inputted into thesecond semiconductor layer 140.

One embodiment of a method for making the LED 80 includes the followingsteps:

-   -   S1010, providing the substrate 110 having the epitaxial growth        surface 112;    -   S1020, growing the buffer layer 116 on the epitaxial growth        surface 112, the first semiconductor layer 120 on the surface of        the buffer layer 116, the active layer 130 on the surface of the        first semiconductor layer 120, and the second semiconductor        layer 140 on the surface of the active layer in sequence;    -   S1030, forming the plurality of three-dimensional        nano-structures 143 by etching the surface of the second        semiconductor layer 140 away from the substrate 110;    -   S1040, forming the third optical symmetric layer 150 on the        surface of the second semiconductor layer 140 away from the        substrate 110, the refractive index of the third optical        symmetric layer 150 being in a range from about 1.2 to about        1.5;    -   S1050, locating the metallic layer 160 on the surface of the        third optical symmetric layer 150 away from the substrate 110;    -   S1060, disposing the fourth optical symmetric layer 170 on the        surface of the metallic layer 160 away from the substrate 110,        the refractive index of the fourth optical symmetric layer 170        being in a range from about 1.2 to about 1.5;    -   S1070, disposing the first optical symmetric layer 180 on the        surface of the fourth optical symmetric layer 170, and the        second optical symmetric layer 190 on the surface of the first        optical symmetric layer 180 in sequence, the difference Δn₁        between the refractive index n₁ of the first optical symmetric        layer 180 and the effective refractive index n₂ of the source        layer and the buffer layer 116 being less than or equal to 0.3,        wherein Δn₁=|n₁−n₂|, the difference Δn₂ between the refractive        index n₃ of the second optical symmetric layer 190 and the        refractive index n₄ of the substrate 110 is smaller or equal to        0.1, and Δn₂=|n₃−n₄|; and    -   S1080, applying the first electrode 124 to be electrically        connected with the first semiconductor layer 120, and applying        the second electrode 144 to be electrically connected with the        second semiconductor layer 140.

The method for making the LED 80 is similar to the method for making theLED 60 except for the step of forming the plurality of three-dimensionalnano-structures 143.

In step S1030, the plurality of three-dimensional nano-structures 143can be formed by the following substeps:

-   -   S1031, locating a mask layer on the surface of the second        semiconductor layer 140;    -   S1032, patterning the mask layer by a nanoimprinting and etching        method;    -   S1033, patterning the surface of the second semiconductor layer        140 by an etching method to form a plurality of        three-dimensional nano-structure preforms; and    -   S1034, forming the plurality of the three-dimensional        nano-structures 143 by removing the mask layer.

In step S1031, the mask layer can be a single layered structure or amulti-layered structure. In one embodiment, the mask layer is themulti-layered structure including a first mask layer and a second masklayer disposed on a surface of the first mask layer. The first masklayer and the second mask layer are stacked on the surface of the secondsemiconductor layer 140 in sequence. A material of the first mask layeris ZEP520A which is developed by Zeon Corp of Japan, a material of thesecond mask layer is HSQ (hydrogen silsesquioxane).

In step S1032, the mask layer can be patterned by the following steps:

-   -   S10321, providing a patterned template which includes a        plurality of protruding structures spaced from and substantially        parallel with each other, and a slot defined between two        adjacent protruding structures;    -   S10322, attaching the template on the second mask layer,        pressing the template at a room temperature, and removing the        template to form a plurality of slots on the second mask layer;    -   S10323, removing the residual second mask layer in the bottom of        the slot to expose the first mask layer, and    -   S10324, patterning the mask layer by removing one part of the        first mask layer corresponding with the slots to expose the        second semiconductor layer 140.

In step S1033, the second semiconductor layer 140 can be placed in aninductively coupled plasma device and etched by an etching gas. In oneembodiment, the etching gas is a mixed gas. The mixed gas can includeCl₂, BCl₃, O₂, and Ar. A power of the inductively coupled plasma deviceranges from about 10 watts to about 100 watts, a flow speed of theetching gas ranges from about 8 sccm to about 150 sccm, a pressure ofthe etching gas can range from about 0.5 Pa to about 15 Pa, and anetching time can range from about 5 seconds to about 5 minutes. In oneembodiment, the flow speed of the Cl₂ is about 26 sccm, the flow speedof the BCl₃ is about 16 sccm, the flow speed of the O₂ is about 20 sccm,and the flow speed of the Ar is about 10 sccm.

More specifically, the second semiconductor layer 140 can be etched bythe following steps:

-   -   S10331, forming a plurality of grooves with the same depth by        etching the surface of second semiconductor layer 140 with the        etching gas;    -   S10332, continuing the etching process so that every two        adjacent protruding structures begin to slant face to face to        form a protruding pair; and    -   S10333, further continuing the etching process so that the two        adjacent protruding structures gradually slant until the tops of        the two adjacent protruding structures contact each other.

In step S10332, the etching gas etches the exposed surface of the secondsemiconductor layer 140 to form the plurality of grooves. The grooveshave substantially the same depth because of the same etching speed.

In step S10332, during the etching process, the etching gas will reactwith the exposed second semiconductor layer 140 to form a protectivelayer. The protective layer will reduce the etching speed of the secondsemiconductor layer 140, and the width of the grooves will slowlydecrease from the outer surface of the second semiconductor layer 140 tothe bottom of the grooves. Thus, the inner wall of the grooves will notbe absolutely perpendicular to the surface of the second semiconductorlayer 140, but form an angle. The etching gas not only etches the secondsemiconductor layer 140, but also etches the top of the protrudingstructures. The width of the top of the protruding structures willdecrease. The resolution of the mask layer will not be affected becausethe speed of etching the top of the protruding structures is muchsmaller than that of the second semiconductor layer 140. Furthermore,every two adjacent protruding structures 1031 will slant face to face.

In step S10333, the tops of the two adjacent protruding structures 1031will gradually approach to each other. The speed of etching the secondsemiconductor layer 140 corresponding to these two closed adjacentprotruding structures 1031 will decrease, and the width of the grooveswill gradually decrease from the outer surface of the secondsemiconductor layer 140 to the bottom of the grooves of the secondsemiconductor layer 140. Because the two adjacent protruding structuresslant face to face to form the protruding pair, the speed of etching thesecond semiconductor layer 140 corresponding to the protruding pair willfurther decrease. Eventually, the tops of the two adjacent protrudingstructures will contact each other, and the etching gas can no longeretch the second semiconductor layer 140 corresponding to the twoadjacent protruding structures. Thus, the first grooves 1136 are formedon the surface of the second semiconductor layer 140. But between everytwo adjacent protruding pairs, the etching speed will change less than aslant speed of the two adjacent protruding structures. Thus, the secondgrooves 1138 are formed, and the depth of the second grooves 1138 willbe greater than that of the first grooves 1136. The plurality ofthree-dimensional nano-structure preforms are obtained.

In step S1034, the three-dimensional nano-structures 143 can be obtainedby dissolving the mask layer. The mask layer can be dissolved in astripping agent such as tetrahydrofuran (THF), acetone, butanone,cyclohexane, hexane, methanol, or ethanol. In one embodiment, thestripping agent is butanone, and the mask layer is dissolved in butanoneand separated from the second semiconductor layer 140.

The plurality of three-dimensional nano-structures 143 can also beformed on the surface of the active layer 130 away from the substrate110 or the surface of the first semiconductor layer 120 away from thesubstrate 110 by the above method. Other three-dimensionalnano-structures 143 above the three-dimensional nano-structures 143 ofthe second semiconductor layer 140 can be formed by directly growing.

In the method for making the LED 80, the nanoimprinting and etchingmethod is used to form the plurality of three-dimensionalnano-structures 143. The nanoimpriting process can be conducted in aroom temperature and the template can be directly used without beingpre-treated. Therefore, the method has a simple process and low cost. Inaddition, a large area array of the plurality of M-shapedthree-dimensional nano-structures 143 can be fabricated because that thetwo adjacent protruding structures of the mask layer can be contactedwith each other by the gas etching to form the plurality of protrudingpairs. Therefore, a yield of the LED 80 can be increased.

Referring to FIG. 13, one embodiment of an LED 90 is provided. The LED90 is substantially the same as the LED 60, except that the LED 90further includes a reflective element 192 disposed on a surface of thesecond optical symmetric layer 190 away from the substrate 110.

The emergent light from the second optical symmetric layer 190 can bereflected back to the substrate 110 by the reflective element 192 toextract the light from the substrate 110. The reflective element 192 canbe a layered structure directly contacting the second optical symmetriclayer 190. The reflective element 192 can be a continuous layer formedby a metal material. The material of the reflective element 192 can beAl, Au, Cu, Ag, or an alloy thereof. A thickness of the reflectiveelement 192 is selected to reflect as much emergent light from thesecond optical symmetric layer 190 as possible. The thickness of thereflective element 192 can be greater than 20 micrometers. In oneembodiment, the material of the reflective element 192 is Ag, thethickness is about 20 nanometers, and the reflective element 192 isformed on the surface of the second optical symmetric layer 190 by avacuum evaporation method or a magnetron sputtering method.

In addition, the reflective element 192 also can be a plurality ofmicro-structures disposed on the surface of the second optical symmetriclayer 190 away from the substrate 110. The plurality of micro-structurescan be grooves or protruding structures. The plurality ofmicro-structures can be at least one of V-shaped, cylindrical,semiorbicular, and pyramid-shaped with or without tips. The plurality ofmicro-structures are uniformly disposed on the surface of the secondoptical symmetric layer 190. The reflective element 192 further caninclude the reflective material disposed on a surface of each of theplurality of micro-structures.

The reflective element 192 disposed on the surface of the second opticalsymmetric layer 190 can make all lights generated from the active layer130 extract from the substrate 110.

The reflective element 192 also can be disposed on the surface of thesubstrate 110 away from the active layer 130, so that all lights extractfrom the surface of the second optical symmetric layer 190.

All lights can extract from one surface of the LED 90 by disposing thereflective element 192. Therefore, a light intensity of the LED 90 canbe increased.

Referring to FIG. 14, one embodiment of an LED 92 includes the firstsemiconductor layer 120, the active layer 130, the second semiconductorlayer 140, the third optical symmetric layer 150, the metallic layer160, the fourth optical symmetric layer 170, the first optical symmetriclayer 180, the first electrode 124, and the second electrode 144. Thefirst semiconductor layer 120 includes a first surface 121 and a secondsurface 122 opposite to the first surface 121. The active layer 130, thesecond semiconductor layer 140, the third optical symmetric layer 150,the metallic layer 160, the fourth optical symmetric layer 170, and thefirst optical symmetric layer 180 are stacked on the second surface 122in the listed sequence. The first electrode 124 is disposed and coversthe first surface 121. The second electrode 144 is electricallyconnected with the second semiconductor layer 140. The differencebetween the refractive index between the first optical symmetric layer180 and the refractive index of the source layer is less than or equalto 0.3.

In the LED 92, the first electrode 124 and the second electrode 144 arepositioned on two opposite sides of the first semiconductor layer 120,and the first electrode 124 covers a major part of the first surface121. A current perpendicularly passes through the second semiconductorlayer 140 of P-type with a high resistance.

The LED 92 is similar to the LED 60, except that there is no secondoptical symmetric layer 190, buffer layer 116, and substrate 110 in theLED 92, and the first electrode 124 covers the whole first surface 121of the first semiconductor layer 120.

In the LED 92, the first electrode 124 and the second electrode 144 areperpendicularly disposed and faced with each other. This structure canproduce a small amount of heat in the second semiconductor layer 140 asthe P-typed semiconductor when applied the current. In addition, thesapphire as the substrate 110 has a poor heat dissipation. Therefore,the heat dissipation of the LED 92 can be improved without the substrate110 of sapphire, and a lifespan of the LED 92 can be improved.

One embodiment of a method for making the LED 92 includes the followingsteps:

-   -   S1310, providing the substrate 110 with the epitaxial growth        surface 112;    -   S1320, growing the buffer layer 116 on the epitaxial growth        surface 112, the first semiconductor layer 120 on the surface of        the buffer layer 116, the active layer 130 on the surface of the        first semiconductor layer 120, and the second semiconductor        layer 140 on the surface of the active layer in sequence;    -   S1330, forming the third optical symmetric layer 150 on the        surface of the second semiconductor layer 140 away from the        substrate 110;    -   S1340, locating the metallic layer 160 on the surface of the        third optical symmetric layer 150 away from the substrate 110;    -   S1350, disposing the fourth optical symmetric layer 170 on the        surface of the metallic layer 160 away from the substrate 110;    -   S1360, disposing the first optical symmetric layer 180 on the        surface of the fourth optical symmetric layer 170, except that        the difference between the refractive index of the first optical        symmetric layer 180 and the refractive index of the source layer        is less than or equal to 0.3;    -   S1370, exposing the first surface 121 of the first semiconductor        layer 120 by removing the substrate 110 and the buffer layer        116; and    -   S1380, locating the first electrode 124 on the first surface 121        and locating the second electrode 144 electrically connected        with the second semiconductor layer 140.

The method for making the LED 92 is substantially the same as the methodof making the LED 60, except that the substrate 110 and the buffer layer116 are removed to expose the first surface 121 of the firstsemiconductor layer 120, there is no need to dispose the second opticalsymmetric layer 190, and the electrode 124 is disposed to cover thewhole first surface 121 in the method of making the LED 92.

In step S1370, the substrate 110 can be removed by methods of laserirradiating, corroding, and self stripping by temperature differences.

The laser irradiating method for removing the substrate 110 includes thefollowing substeps:

-   -   S1371, polishing and washing the surface of substrate 110 away        from the first semiconductor layer 120;    -   S1372, placing the substrate 110 on a platform and irradiating        the polished surface of the substrate 110 by the laser; and    -   S1373, immersing the irradiated substrate 110 in a solution to        remove the substrate 110.

In step S1371, the surface of the substrate 110 away from the firstsemiconductor layer 120 can be polished by mechanical polishing orchemical polishing. The polished surface becomes smooth to decrease ascattering of the laser irradiating. The polished surface can be washedby a chlorhydric acid or a sulfuric acid to remove metal impurities,grease, and dirt.

In step S1372, an irradiating direction is substantially perpendicularto the polished surface. The laser can access the substrate 110 andreach to the first semiconductor layer 120 to strip the substrate 110from the first semiconductor layer 120. An energy of the laser issmaller than a band gap energy of the substrate 110 and greater than theband gap energy of the first semiconductor layer 120. The buffer layer116 can strongly absorb the laser energy, thereby results adecomposition thereof due to a rapidly increasing temperature. In oneembodiment, the material of the first semiconductor layer 120 is the GaNand the band gap energy is about 2.2 electron volts (ev), the materialof the substrate 110 is the sapphire and the band gap energy is about9.9 ev; the material of the buffer layer 116 is the low temperature GaN,the laser is a KrF laser, a laser wavelength of the KrF laser is about248 nanometers and the energy is about 5 ev, a pulse width of the KrFlaser is in a range from about 20 nanoseconds to about 40 nanoseconds,an energy density is about 400 mJ/cm² to about 600 mJ/cm², a spot shapeis rectangle, a size of the spot is about 0.5 micrometers×0.5micrometers, the laser scans starting from an edge of the polishedsurface, and a scanning step is about 0.5 mm/s. The low temperature GaNis decomposed to a Ga and N₂.

The buffer layer 116 has a strong laser absorption which results in therapidly increasing temperature, thus the buffer layer 116 is decomposed.The first semiconductor layer 120 has weak laser absorption. Therefore,the laser will not damage the first semiconductor layer 120.

The process of the laser irradiating is conducted in a vacuum or anatmosphere filled with a protecting gas. The protecting gas can benitrogen, helium, argon, or combinations thereof.

In step S1373, the irradiated substrate 110 can acidize by immersing inan acid solution. The acid solution can dissolve the decomposed Ga fromthe buffer layer 116. Therefore, the substrate 110 is stripped from thefirst semiconductor layer 120. The acid solution can be a hydrochloricacid, nitric acid, or a sulfuric acid.

Referring to FIG. 15, one embodiment of an LED 93 is provided. The LED93 is similar to the LED 92, except that the LED 93 includes theplurality of three-dimensional nano-structures 143. An arrangement ofthe plurality of three-dimensional nano-structures 143 of the LED 93 aresubstantially the same as the arrangement of the plurality ofthree-dimensional nano-structures 143 in the semiconductor structure 80except that there is no second optical symmetric layer 190 in the LED93. More specifically, the plurality of three-dimensionalnano-structures 143 can be formed on at least one layer of the firstsemiconductor layer 140, the third optical symmetric layer 150, themetallic layer 160, the fourth optical symmetric layer 170, and thefirst optical symmetric layer 180 of the LED 93. In one embodiment, theplurality of three-dimensional nano-structures 143 are formed on eachlayer of the first semiconductor layer 140, the third optical symmetriclayer 150, the metallic layer 160, the fourth optical symmetric layer170, and the first optical symmetric layer 180 of the LED 93.

One embodiment of a method for making the LED 93 includes the followingsteps:

-   -   S1510, providing the substrate 110 with the epitaxial growth        surface 112;    -   S1520, growing the buffer layer 116 on the epitaxial growth        surface 112, the first semiconductor layer 120 on the surface of        the buffer layer 116, the active layer 130 on the surface of the        first semiconductor layer 120, and the second semiconductor        layer 140 on the surface of the active layer in sequence;    -   S1530, forming the plurality of three-dimensional        nano-structures 143 by etching the surface of the second        semiconductor layer 140 away from the substrate 110;    -   S1540, forming the third optical symmetric layer 150 on the        surface of the second semiconductor layer 140 away from the        substrate 110;    -   S1550, forming the metallic layer 160 on the surface of the        third optical symmetric layer 150 away from the substrate 110;    -   S1560, disposing the fourth optical symmetric layer 170 on the        surface of the metallic layer 160 away from the substrate 110;    -   S1570, disposing the first optical symmetric layer 180 on the        surface of the fourth optical symmetric layer 170, the        difference between the refractive index of the first optical        symmetric layer 180 and the refractive index of the source layer        being less than or equal to 0.3;    -   S1580, exposing the first surface 121 of the first semiconductor        layer 120 by removing the substrate 110; and        S1590, locating the first electrode 124 on the first surface 121        and locating the second electrode 144 electrically connected        with the second semiconductor layer 140

The method for making the LED 93 is similar to the method for making theLED 92 except that the plurality of three-dimensional nano-structures143 in the LED 93 are formed. The process for forming the plurality ofthree-dimensional nano-structures 143 in the LED 93 is substantially thesame as the process in making the semiconductor structure 80.

Referring to FIG. 16, one embodiment of an LED 94 is provided. The LED94 is substantially the same as the LED 92, except that the LED 94further includes the reflective element 192 disposed on a surface of thefirst optical symmetric layer 180 away from the first semiconductorlayer 120.

The emergent light generated from the active layer 130 can be reflectedback to the first electrode 124, thereby the emergent lights can beconcentrate extracted from the first electrode 124. Therefore, the LED94 has a good light extraction efficiency.

Referring to FIG. 17, one embodiment of a solar cell 200 includes afirst collecting electrode 32, the substrate 110, the buffer layer 116,the first silicon layer 126, a photovoltaic layer 136, a second siliconlayer 146, the third optical symmetric layer 150, the metallic layer160, the fourth optical symmetric layer 170, the first optical symmetriclayer 180, the second optical symmetric layer 190, and a secondcollecting electrode 34. The substrate 110, the buffer layer 116, thefirst silicon layer 126, the photovoltaic layer 136, the second siliconlayer 146, the third optical symmetric layer 150, the metallic layer160, the fourth optical symmetric layer 170, the first optical symmetriclayer 180, the second optical symmetric layer 190, and the secondcollecting electrode 34 are stacked on a surface of the first collectingelectrode 32 in the listed sequence. The refractive index of the thirdoptical symmetric layer 150 is substantially the same as the refractiveindex of the fourth optical symmetric layer 170. The first silicon layer126, the photovoltaic layer 136, and the second silicon layer 146constitutes a second source layer. A difference between the refractiveindex of the first optical symmetric layer 180 and a second effectiverefractive index of the second source layer and the buffer layer 116 isless than or equal to 0.3. The refractive index difference between thesecond optical symmetric layer 190 and the substrate 110 is less than orequal to 0.1. The solar cell 200 is an optical symmetric structure withthe metallic layer 160 as the optical symmetric center. A structure ofthe solar cell 200 is similar to the semiconductor structure 10.

The substrate 110, the buffer layer 116, the third optical symmetriclayer 150, the fourth optical symmetric layer 170, and the secondoptical symmetric layer 190 of the solar cell 200 are optional elements.The substrate 110, the buffer layer 116, and the second opticalsymmetric layer 190 should exist or be absent in the solar cell 200together to ensure the optical symmetric structure of the solar cell.Similarly, the third optical symmetric layer 150 and the fourth opticalsymmetric layer 170 should exist or be absent in the solar cell 200together to ensure the optical symmetric structure of the solar cell.

A semiconductor type between the first silicon layer 126 and the secondsilicon layer 146 are opposite. The first silicon layer 126 can beN-typed or P-typed. In one embodiment, the first silicon layer 126 isP-typed, the second silicon layer 146 is N-typed. A material and athickness of the photovoltaic layer 136 are substantially the same asthe material and the thickness of the semiconductor structure 10.

The first collecting electrode 32 and the second collecting electrode 34can be made with a same material or different materials. The firstcollecting electrode 32 or the second collecting electrode 34 can be ametal plate with a continuous surface. The material of the firstcollecting electrode 32 or the second collecting electrode 34 can be Al,Cu, Ag, or an alloy thereof. A thickness of the first collectingelectrode 32 or the second collecting electrode 34 can be in a rangefrom about 50 nanometers to about 300 nanometers. In one embodiment,both the first collecting electrode 32 and the second electrode 34 arebar-shaped Al foils with the thickness of about 200 nanometers.

The solar cell 200 includes a light-input surface 310 and a light-outputsurface 320. One side of the substrate 110, the buffer layer 116, thefirst silicon layer 126, the photovoltaic layer 136, the second siliconlayer 146, the third optical symmetric layer 150, the metallic layer160, the fourth optical symmetric layer 170, the first optical symmetriclayer 180, and the second optical symmetric layer 190 constitute thelight-input surface 310. An opposite side of the side of the substrate110, the buffer layer 116, the first silicon layer 126, the photovoltaiclayer 136, the second silicon layer 146, the third optical symmetriclayer 150, the metallic layer 160, the fourth optical symmetric layer170, the first optical symmetric layer 180, and the second opticalsymmetric layer 190 constitute the light-output surface 320 of the solarcell 200.

Sunlight irradiates the light-input surface 310 and reaches the metalliclayer 160. The metallic plasma is then generated from the metallic layer160. The metallic plasma is absorbed by a P-N conjunction formed by thefirst silicon layer 126 and the second silicon layer 136 to form largeramounts of electrons and holes. The electrons move to the secondcollecting electrode 34 and the holes move to the first collectingelectrode 32 to form a current.

Referring to FIG. 18, one embodiment of a waveguide 400 is provided. Thewaveguide 400 is substantially the same as the semiconductor structure10. The metallic layer 160 of the waveguide 400 includes a first side161 and a substantially parallel and opposite second side 162.Electromagnetic waves can enter from the first side 161 and pass throughfrom the second side 162. The metallic plasma is generated when theentered electromagnetic waves reaches the metallic layer 160. Themetallic plasma carrying information of the electromagnetic wavesspreads in the metallic layer 160 and then is converted back to theelectromagnetic waves when reaching the second side 163. Therefore, thewaveguide 400 can conduct the electromagnetic waves. In addition, thewaveguide 400 is the optical symmetric structure, the metallic plasmacan be restricted in the metallic layer 160 and uniformly conducted inthe metallic layer 160.

Finally, it is to be understood that the above-described embodiments areintended to illustrate rather than limit the present disclosure.Variations may be made to the embodiments without departing from thespirit of the present disclosure as claimed. Elements associated withany of the above embodiments are envisioned to be associated with anyother embodiments. The above-described embodiments illustrate the scopeof the present disclosure but do not restrict the scope of the presentdisclosure.

Depending on the embodiment, certain of the steps of methods describedmay be removed, others may be added, and the sequence of steps may bealtered. It is also to be understood that the description and the claimsdrawn to a method may include some indication in reference to certainsteps. However, the indication used is only to be viewed foridentification purposes and not as a suggestion as to an order for thesteps.

What is claimed is:
 1. A method for making a light emitting diodecomprising: providing a substrate having an epitaxial growth surface;growing a buffer layer on the epitaxial growth surface, a firstsemiconductor layer on a surface of the buffer layer, an active layer ona surface of the first semiconductor layer, and a second semiconductorlayer on a surface of the active layer in sequence, wherein the firstsemiconductor layer, the active layer, and the second semiconductorlayer cooperatively constitutes a source layer; disposing a thirdoptical symmetric layer on a surface of the second semiconductor layeraway from the substrate, wherein a refractive index of the third opticalsymmetric layer is in a range from about 1.2 to about 1.5; forming ametallic layer on a surface of the third optical symmetric layer awayfrom the substrate; disposing a fourth optical symmetric layer on asurface of the metallic layer away from the substrate, wherein arefractive index of the fourth optical symmetric layer is in a rangefrom about 1.2 to about 1.5; forming a first optical symmetric layer ona surface of the fourth optical symmetric layer away from the substrate,wherein a refractive index difference between the first opticalsymmetric layer and the source layer is smaller than or equal to 0.3;exposing the first semiconductor layer by removing the substrate and thebuffer layer; and applying a first electrode on an exposed surface ofthe first semiconductor layer, and applying a second electrodeelectrically connected with the second semiconductor layer.
 2. Themethod of claim 1, wherein a material of the metallic layer is selectedfrom the group consisting of gold, silver, aluminum, copper, and analloy thereof.
 3. The method of claim 1, wherein a thickness of themetallic layer is in a range from about 10 nanometers to about 30nanometers.
 4. The method of claim 1, wherein a thickness of the secondsemiconductor layer is in a range from about 5 nanometers to about 20nanometers.
 5. The method of claim 1, wherein the refractive index ofthe third optical symmetric layer is substantially the same as therefractive index of the fourth optical symmetric layer.
 6. The method ofclaim 1, wherein a thickness of the third optical symmetric layer is ina range from about 5 nanometers to about 40 nanometers and a thicknessof the fourth optical symmetric layer is in a range from about 5nanometers to about 40 nanometers.
 7. The method of claim 1, wherein athickness of the first optical symmetric layer is substantially the sameas a thickness of the source layer.
 8. The method of claim 1 furthercomprising forming a plurality of three-dimensional nano-structures onthe surface of the second semiconductor layer away from the substratebefore the step of disposing the third optical symmetric layer.
 9. Themethod of claim 8, the plurality of three-dimensional nano-structuresare formed by the following steps: locating a patterned mask layer onthe surface of the second semiconductor layer away from the substrate,wherein the patterned mask layer comprises a plurality of protrudingstructures spaced from and substantially parallel to each other, aplurality of slots are defined between every two adjacent protrudingstructures of the plurality of protruding structures, and the secondsemiconductor layer is exposed via the plurality of slots; etching theexposed second semiconductor layer to form a plurality of grooves andtops of the every two adjacent protruding structures contact each otheras a plurality of protruding pairs; and removing the patterned masklayer so that the plurality of three-dimensional nano-structures of thesurface of the second semiconductor layer away from the substrate areM-shaped.
 10. The method of claim 9, wherein an etching speed to thesecond semiconductor layer in the plurality of protruding pairs issmaller than the etching speed to the second semiconductor between theplurality of protruding pairs.
 11. The method of claim 9, wherein aplurality of first grooves are formed in the plurality of protrudingpairs, a plurality of second grooves are formed between the plurality ofprotruding pairs, and a depth of the plurality of first grooves issmaller than a depth of the second grooves after the etching to form theplurality of three-dimensional M-shaped nano-structures.
 12. The methodof claim 8 further comprising forming the plurality of three-dimensionalnano-structures on at least one surface of the third optical symmetriclayer, the metallic layer, the fourth optical symmetric layer, and thefirst optical symmetric layer.